Denitrification is a microbial process where microorganisms use nitrate, which is a form of nitrogen that is biologically available, and they convert it through back to nitrogen gas. So it is essentially the completion of the nitrogen cycle. In some ways, it’s the opposite to nitrogen fixation. So nitrogen fixation takes nitrogen gas out of the atmosphere and makes it biologically available, whereas denitrification converts that nitrogen back out of the soil back to nitrogen gas, completing the nitrogen cycle.
Professor Louis Schipper, University of Waikato